Method for impregnating a carbonaceous brush with silver



Apnl 26, 1960 D. RAMADANOFF 2,934,460

METHOD FOR IMPREGNATING A CARBONACEOUS BRUSH WITH SILVER Filed March 31,1958 H I'rI-I IMPREGNATE BRUSH wrrI-I wE i E ESE Y PRECIPITANT (e.g.,sIIvER NITRATE) AND PRECIPITANT e.g., POLYGLYCOL) g POLYGLYCOL) IMPREGNATEBRUSH WITH SOLUBLE SILVER SALT (eg.,sII vER NITRATE) ALTERNATE PROCEDUREL- HEAT TO PRECIPITATE SILVER COMPLEX IN srru (APPROX.I50C.)

MAINTAIN HEAT TO DRY BRUSH HEAT TO REDUCE SILVER COMPLEX TO METALLICSILVER (APPROX. 450 c) IMPREGNATE BRUSH WITH SOLUBLE SULFUR COMPOUND(e.g,AMMoNIuM POLYSULFIDE) HEAT TO CONVERT METALLIC SILVER TO SULFIDEAND DRIVE OFF EXCESS SULFUR INVENTOR DIMITER RAMADANOFF BY Q M ATTORNEYUnited States METHOD FOR IMPREGNATING A CARBONA- CEOUS aiwsn WITH srrvnnDitniter Ramadanoif, Berea, Ohio, assignor to Union Carbide Corporation,a corporation of New York Application March 31, 1958, Serial No. 725,116

8 Claims. '(Cl. 117228) a method of impregnating carbonaceous brusheswith silver and silver sulfide.

impregnation of carbon and graphite brushes with silver or silversulfide has beenfoundto benefit the operation of the brushes when incurrent-carrying contact with a steel ring or commutator of adynamo-electric apparatus. Specifically, impregnation with silver orsilver sulfidegenerally increases-the life of the brushes andbeneficially alters the voltage drop across the contact between thebrushes and the ring or commutator. Of equal importance, silver orsilver sulfide impregnated within a brush has been found to reduce theoperating temperature of the brush and to enhance its frictionalproperties.

Heretofore, it has been. the practice to incorporate silver in acarbonaceous brush by impregnating the brush with a soluble compound ofsilver, such as silver nitrate, and decomposing the silver nitrate tosilver in situ, usual- 1y, by baking the brushes. This practice has thedisadvantage that water contained within the brush must be removed andthat this is normally accomplished upon baking, in which process theimpregnant migrates to the brush surface. Silver sulfide has beenproduced in situ by prior processes by first impregnating brush stockwith silver nitrate and then converting the nitrate to the sulfide.Here, too, migration of impregnant may be encountered.

Migration of the impregnant silver or silver sulfide is undesirablebecause it renders the structure of a brush non-homogeneous, the silveror silver sulfide additive forming a shell so to speak at the surface ofthe brush. In operation, this is disadvantageous for a number ofreasons. For one, migration may alter the current-carryingcharacteristics of a brush, and for another, the frictional propertiesof a brush may not be as good as otherwise possible if silver or silversulfide were uniformly distributed throughout the brush.

A solutionto the above problem of migration is set forth in my copendingapplication, Serial No. 533,021, filed September 7, 1955, now U.S.Patent No. 2,847,332. As described in that application, the problem issolved by impregnating a carbonaceous brush with an aqueous solution ofsilver nitrate, precipitating the silver ions out of the silver nitrateas an insoluble silver compound while the brush is still wet to preventmigration of the silver nitrate, and then heating the carbonaceous brushto reduce the insoluble silver compound to metallic silver.The'insoluble compound formed in the carbonaceous brush,,,in accordancewith the principles of the invention, would have a low volatility, andshould be capable on thermal decomposition of yielding reaction productswhich are not'deleterious to the composition of the carbonaceous brush.To this end the carbonaceous brush may be impregnated, by the alternateapplication of vacuum and pressure, with an aqueous solution of silvernitrate.

with silver or silver sulfide.

migration commences, it may be exposed to ammonia fumes to precipitatethe silver ions in situ as silver oxide, or, preferably, the silver ionsmay be precipitated in stiu as silver carbonate by placlng thecarbonaceous brush, while still in a wet state, in an aqueous saturatedsolution of ammonium carbonate. The silver oxide or silver carbonate isthen reduced to silver by heat treatment.

The disadvantage of using this method is that the precipitation stage istime consuming and is limited by the rate of diffusion of theprecipitant. The precipitation time is approximately 15 hours.

. One object of the invention is to provide a new and novel method fortreating carbonaceous brushes with silver.

Another object is to provide a method whereby a soluble compound ofsilver is impregnated within a brush and rapidly precipitated in situ bythe application of heat, thereby prohibiting migration and moreuniformly distributing the silver within the brush. I

A more specific object is to provide a method for impregnating acarbonaceous brush with silver or silver sulfide by impregnating thebrush with a soluble silver compound and effecting rapid precipitationof the soluble silver compound in situ, removing contained water byevaporation without migration, and reducing the precipitated silvercompound to metallic silver which then may be converted to silversulfide if desired.

Still another object is to provide a method which further insures a moreuniform distribution of silver or silver sulfide by first introducingthe precipitant within the brush before impregnation with an aqueoussolution of a soluble silver compound.

. The single figure in the drawing is a flow diagram illustrating themethod of the present invention, using the 7 preferred impregnauts.

According to the invention, it has been found that a carbonaceous brushmay be impregnated with silver or silver sulfide with a uniformdistribution by impregnating the brush with an aqueous solution of asoluble compound of silver, such as silver nitrate, and a suitableprecipitant; precipitating the silver compound rapidly in situ andheating to a temperature sufficient to remove contained water in thebrush, and subsequently firing the brush at a sufiicient temperature toreduce the silver compound to metallic silver, thereby also evolving anyorganic matter present within the brush. Thereafter the thus-impregnatedbrush may be treated suitably to convert the metallic silver in situ tosilver sulfide, a suitable method being by subjecting the brush to asolution of a soluble compound of sulfur such as ammonium polysulfide,drying at an elevated temperature, and firing the brush at a temperaturesufficient to convert the impregnated metallic silver to silver sulfide.i

In the practice of the invention, a suitable precipitant may be asolution of a compound or mixture of compounds soluble in water and inwhich silver nitrate when heated to elevated temperatures issubstantially insoluble. Acetone, an alcohol such as methyl or ethylalcohol, or ethylene diamine are good precipitants. more preferredprecipitants are the polyglycols. For example, the polyoxyethylenes orpolyoxypropylenes are very good-precipitants in practicing the method ofthe invention. The most desirable precipitant to be used is an equalmixture of polyethylene oxide and polypropylene oxide.

The percentage of silver nitrate and the precipitant to be used aregenerally not too critical, those skilled in the art being well versedin the amounts of silver nitrate solution to be used for desiredimpregnation of carbon The For example, it is known" that to impregnatean electrographitic brush with 8 percent to 12 percent by weight ofsilver or silver sulfide within the brush, a solution of about 30percent to 40 percent by weight of silver nitrate in water should beused. To this solution, it has been found that about percent by volumeof the precipitant; say 50 percent ethylene oxide and 50 percentpropylene oxide, is a suitable mixture. More or less precipitant can beused effectively. The amount of the sulfur-containing solution used toconvert the impregnated silver in the brush to silver sulfide should beat least sufiicient to combine with the amount of silver deposited. Moresolution used than required will provide an excess of .sulfur which isdriven-off by baking. The removal of excess sulfur is important since itcan otherwise increase brush friction and contact voltage drop.

Illustratively, the method of the invention may best be explained by thefollowing example: A natural graphite brush was impregnated, byalternate application of vacuum and pressure, with a solution of 33.3percent of silver nitrate in water containing about 10 percent .byvolume of the polyglycol precipitant, 50 percent ethylene oxide and 50percent propylene oxide. The brush was then heated to an elevatedtemperature of about 150 C. and the impregnated silver nitrate wasprecipitated almost immediately in situ without migration, and thistemperature was maintained for one hour to dry the brush. Subsequently,the brush was fired at a temperaure of about 400 C. for four minutes andthe silver nitrate was thereby reduced to metallic silver. Any organicmaterial remaining from the precipitant was driven-off upon firing. Inasecond step, the brush was similarly impregnated with an ammoniumpolysulfide solution, dried at 150 C. for one hour and fired at atemperature of 450 C. to convert the metallic silver impregnated in thebrush to silver sulfide, while also removing any free sulfur present.

In another example, it was found that the properties of the brush of theabove example could be improved by altering the initial silver nitrateimpregnation. In this example, a natural graphite brush was firstimpregnated with the polyglycol precipitant in an amount equivalent to10 percent by volume of silver nitrate to be later impregnated.Thereafter, the brush was dried and then impregnated with a solution of33.3 percent by volume of silver nitrate in water and the brush furthertreated as explained in the above example. Referring to the tablesbelow, this brush in operation exhibited a lower contact voltage dropand a slightly improved wear life.

The following tables illustrate the improved properties achieved byimpregnating a brush with silver and silver sulfide according to themethod of the invention. In one series of tests, an unimpregnatednatural graphite brush and the same brush impregnated withsilver sulfideby prior art methods were compared with two brushes impregnatedaccording to the invention, one impregnated with silver and another withsilver sulfide. Table I summarizes the results of this test from whichthe bene-' fits of the invention are readily apparent.

TABLE I High speed tests on steel rings [Current density 120a.p.s.1.;rub3bi)ng spe]ed 14,150 t.p.m. spring pressure D.S.l.

The figure of Table I show that a brush impregnated with silver sulfideby the method of the invention has a lower contact drop, an improvedcoefficient of friction, a much lower brush temperature, and a greatlyimproved life as compared to a brush impregnated with silver sulfide byprior methods. Moreover, a brush impregnated with silver by the methodof the invention has a lower contact drop, a tremendously greater life,and a lower brush temperature than an unimpregnated brush and only aslightly higher contact drop and shorter life than a brush impregnatedwith silver sulfide'by prior methods and does have a lower coefficientof friction and a much lower brush temperature than such brush.

In another test, a natural graphite brush was treated by incorporatingsilver powder within the brush and another brush treated by convertingthe powdered silver to silver sulfide. These brushes were compared inthe test with two brushes impregnated with silver sulfide according tothe invention, one of the brushes being prepared by first impregnatingthe brush with the polyglycol .precipitant as in the above example.Table II illustrates the results of the test.

I TABLE II [Current density 120 a.p.s.i.; rubsbng speled 14,150 f.p.m.;spring pressure p.s..

Brush (Natural Graphite Contact Life Ooefii- Brush Test Grade) Drop(Hrs./ cient of Tem Hours (Volts) In.) Friction C.

Powders (10% Ag added In mix) 2. 50 2, 000 0. 15 111 43. 25 Powders (10%Ag changed partly to AgzS) 2. 60 2, 215 0.12 104 43. 08 Impregnatcd(12.91%

AgzS method of the a I lnventiomtnaflzibugu 2.30 3, 425 0.13 103 66. 34mpregnel e 0 2 30 2 940 0 15 110 is 7 Ag2SDll0i impregnation ofprecipitant) 1. 98 3, 535 0. 25 120 65. 75

Probably the most desirable advantage of the invendue to poordistribution of the silver additive, the brush was impregnated with amuch greater quantity of silver nitrate than now required if it wasdesired to impregnate the whole brush with silver or silver sulfide. Bythe invention, it is now possible to achieve a more uniform distributionof silver and silver sulfide in a brush with the use of lesserquantities of the impregnant in a much shorter period of time.

A test was made to evaluate the effect of silver nitrate in situ withina brush before the impregnant was decomposed to silver as by prior artmethods. In the test, two cylinders of an electro-graphitic brush gradewere impregnated with a solution of 33.3 percent of silver nitrate inwater, one of the cylinders being treated with the solution modified bythe addition of 10 percent by vollnne of 50 percent ethylene oxide-50per cent propylene oxide. The cylinders measured 1 /2 inches long and 1inch in diameter. Immediately after impregnation, the brushes wereheated at about C. for one hour to remove contained water. Thereaftenthe cylinders were fired for five minutes at 450 C. to decompose silvernitrate to silver. The cylinders wereweighed and the final pick-up ofsilver calculated. The ends of each cylinder were cut and the cylindersdrilled with a hollow drill /2 inch in diameter, and the center cores ofthe cylinders removed. The outer shell and center core of each cylinderwere separately a shed and the silver pick-up calculated. Table IIIsummarizes the results of the'test from which the beneficial effect ofimpregnation and precipitation of silver nitrate according to theinvention is readily apparent.

precipitating TABLE 111 Evaluation of uniform distribution The abovetables clearly show the success of the subject invention. Whencarbonaceous brushes are treated by prior art methods of impregnatingthe brush with a soluble compound of silver such as silver nitrate anddecomposing the silver nitrate to silver by baking the brushes, themigration of the silver to the outer surface results in non-uniformityof the silver distribution. However, treatments according to the methodof the invention show highly improved uniformity of silver distribution.Also, the present invention provides a method whereby the reduced silverin the brush may be readily converted to silver sulfide.

Also, the present invention provides a method whereby the silver may beuniformly distributed in the carbonaceous brush in a much shorter periodof time than by the method of my copending application, Serial No.533,021, filed September 7, 1955, now U.S. Patent No. 2,847,332. Brushesmade by the method of that invention require a day of processing time ormore whereas brushes made according to the principles of the subjectinvention may be impregnated, dried, and sulfided within anhour.

What is claimed is:

1. A method of introducing and uniformly distributing metallic silver ina carbonaceous brush, which method comprises impregnating the brush withan aqueous solution of silver nitrate and a precipitant selected fromthe group consisting of acetone, methyl alcohol, ethyl alcohol, ethylenediamine, polyglycols, and an equal mixture of ethylene oxide andpropylene oxide, heating the brush to a temperature such that the silvernitrate is precipitated rapidly in situ and the contained water isevaporated, and subsequently further heating the brush to a suflicienttemperature to reduce the silver nitrate to silver, while evolving anyorganic matter present in the brush.

2. The method of claim 1 wherein the impregnated silver is furtherconverted to silver sulfide by impregnating the brush with ammoniumpolysulfide, heating said brush to an elevated temperature to dry saidbrush, and further elevating temperature of said brush to convert saidimpregnated metallic silver to silver sulfide, while also removing anyfree sulfur present.

3. A method of introducing and uniformly distributing metallic silver ina carbonaceous brush which method comprises impregnating the brush witha precipitant sedrying the brush, subsequently impregnating the brushwith an aqueous solution of silver nitrate, heating the brush to rapidlyprecipitate the silver nitrate in situ without migration While containedwater is evaporated, and subsequently further heating the brush to asufiicient temperature to reduce the silver nitrate to silver, whileevolving any organic matter present in the brush.

4. The method of claim 3 wherein the impregnated silver is furtherconverted to silver sulfide by impregnating the brush with ammoniumpolysulfide, raising said brush to an elevated temperature to dry saidbrush, and further elevating the temperature of said brush to completethe conversion of said impregnated metallic silver to silver sulfide,while also removing any free sulfur present.

5. A method of introducing and uniformly distributing metallic silver ina carbonaceous brush, which method comprises impregnating said brushwith an aqueous solution of silver nitrate and a precipitant which is anequal mixture of ethylene oxide and propylene oxide, heating said brushto approximately 150 C. to rapidly precipitate said silver nitrate insitu and to evaporate the contained water, and subsequently furtherheating said brush to approximately 400 C. to reduce said silver nitrateto silver, while evolving any organic matter present.

6. A method of introducing and uniformly distributing metallic silver ina carbonaceous brush which method comprises impregnating said brush witha precipitant which is an equal mixture of ethylene oxide and propyleneoxide and drying said brush, subsequently impregnating said brush withan aqueous solution of silver nitrate, heating said brush toapproximately 150 C. to rapidly precipitate said silver nitrate in situwithout migration while contained water is evaporated, and subsequentlyfurther heating said brush to approximately 400 C. to reduce said silvernitrate to silver, while evolving any organic matter present in saidbrush.

7. The method of claim 5 wherein the impregnated silver is furtherconverted to silver sulfide by impregnating said brush with ammoniumpolysulfide, heating said brush to approximately C. to dry said brush,and further heating said brush to approximately 450 C. to convert saidimpregnated metallic silver to silver sulfide, while also removing anyfree sulfur present.

8. The method of claim 7 wherein the impregnated silver is converted tosilver sulfide by impregnating said brush with ammonium polysulfide,heating said brush to approximately 150 C. to dry said brush and furtherheating said brush to approximately 450 C. to complete the conversion ofsaid impregnated metallic silver to silver sulfide, while also removingany free sulfur present.

References Cited in the file of this patent UNITED STATES PATENTS2,428,036 Peters et al Sept. 30, 1947 2,602,757 Kantrowitz et al July 3,1952 2,748,034 Bobal May 29, 1956 2,847,332 Ramadanofi Aug. 12, 1958FOREIGN PATENTS 777,511 Great Britain June 26, 1957

1. A METHOD OF INTRODUCING AND UNIFORMLY DISTRIBUTING METALLIC SILVER INA CARBONACEOUS BRUSH, WHICH METHOD COMPRISES IMPREGNATING THE BRUSH WITHAN AQUEOUS SOLUTION OF SILVER NITRATE AND A PRECIPITANT SELECTED FROMTHE GROUP CONSISTING OF ACETONE, METHYL ALCOHOL, ETHYL ALCOHOL, ETHYLENEDIAMINE, POLYGLYCOLS, AND AN EQUAL MIXTURE OF ETHYLENE OXIDE ANDPROPYLENE OXIDE, HEATING THE BRUSH TO A TEMPERATURE SUCH THAT THE SILVERNITRATE IS PRECIPITATED RAPIDLY IN SITU AND THE CONTAINED WATER ISEVAPORATED, AND SUBSEQUENTLY FURTHER HEATING THE BRUSH TO A SUFFICIENTTEMPERATURE TO REDUCE THE SILVER NITRATE TO SILVER, WHILE EVOLVING ANYORGANIC MATTER PRESENT IN THE BRUSH.